This invention relates generally to the determination of various downhole parameters of a wellbore penetrated by a subsurface formation. More particularly, this invention relates to the determination of downhole pressures, such as annular pressure and/or formation pore pressure, during a wellbore drilling operation. In a typical drilling operation, a downhole drilling tool drills a borehole, or wellbore, into a rock or earth formation. During the drilling process, it is often desirable to determine various downhole parameters in order to conduct the drilling process and/or learn about the formation of interest.
Present day oil well operation and production involves continuous monitoring of various subsurface formation parameters. One aspect of standard formation evaluation is concerned with the parameters of downhole pressures and the permeability of the reservoir rock formation. Monitoring of parameters, such as pore pressure and permeability, indicate changes to downhole pressures over a period of time, and is essential to predict the production capacity and lifetime of a subsurface formation, and to allow safer and more efficient drilling conditions. Such downhole pressures may include annular pressure (PA or wellbore pressure), pressure of the fluid in the surrounding formation (PP pore pressure), as well as other pressures.
During drilling of oil and gas wells using traditional downhole tools, it is common for the drill string to become stuck against the formation. A common type of sticking, known as differential sticking, occurs when a seal is formed between a portion of the downhole tool and the mudcake lining the formation. The pressure of the wellbore relative to the formation pressure assists in maintaining the seal between the mud cake and the downhole tool, typically when the tool is stationary. The hydrostatic pressure acting on the downhole tool increases the friction and makes movement of the drill pipe difficult or impossible. Monitoring downhole pressure conditions enables detection of the downhole pressure conditions likely to result in differential sticking.
Techniques have been developed to obtain downhole pressure measurements through wireline logging via a “formation tester” tool. This type of measurement requires a supplemental “trip” downhole with another tool, such as a formation tester tool, to take measurements. Typically, the drill string is removed from the wellbore and a formation tester is run into the wellbore to acquire the formation data. After retrieving the formation tester, the drill string must then be put back into the wellbore for further drilling. Examples of formation testing tools are described in U.S. Pat. Nos. 3,934,468; 4,860,581; 4,893,505; 4,936,139; and 5,622,223. These patents disclose techniques for acquiring formation data while the wireline tools are disposed in the wellbore, and in physical contact with the formation zone of interest. Since “tripping the well” to use such formation testers consumes significant amounts of expensive rig time, it is typically done under circumstances where the formation data is absolutely needed, or it is done when tripping of the drill string is done for a drill bit change or for other reasons.
Techniques have also been developed to acquire formation data from a subsurface zone of interest while the downhole drilling tool is present within the wellbore, and without having to trip the well to run formation testers downhole to identify these parameters. Examples of techniques involving measurement of various downhole parameters during drilling are set forth in U.K. Patent Application GB 2,333,308 assigned to Baker Hughes Incorporated, U.S. patent application Ser. No. 6,026,915 assigned to Halliburton Energy Services, Inc. and U.S. Pat. No. 6,230,557 assigned to the assignee of the present invention.
Despite the advances in obtaining downhole formation parameters, there remains a need to further develop techniques which permit data collection during the drilling process. Benefits may also be achieved by utilizing the wellbore environment and the existing operation of the drilling tool to facilitate measurements. FIG. 1 shows a typical drilling system and related environment. A downhole drilling tool 100 is extended from a rig 180 into a wellbore 110 and drilling fluid 120, commonly known as “drilling mud”, is pumped into an annular space 130 between the drilling tool and the wellbore. The drilling mud performs various functions to facilitate the drilling process, such as lubricating the drill bit 170 and transporting cuttings generated by the drill bit during drilling. The cuttings and/or other solids mix within the drilling fluid to create a “mudcake” 160 that also performs various functions, such as coating the borehole wall. Portions of the drilling tool often scrape against the wellbore wall, push away the mudcake and come into direct contact with the wellbore wall. When the drill string stops periodically, as it does when a standoff pipe is added, portions of the drilling tool may also rest against the wellbore wall, and mudcake if present.
The dense drilling fluid 120 conveyed by a pump 140 is used to maintain the drilling mud in the wellbore at a pressure (annular pressure PA) higher than the pressure of fluid in the surrounding formation 150 (pore pressure PP) to prevent formation fluid from passing from surrounding formations into the borehole. In other words, the annular pressure (PA) is maintained at a higher pressure than the pore pressure (PP) so that the wellbore is “overbalanced”(PA>PP) and does not cause a blowout. The annular pressure (PA) must also, however, be maintained below a given level to prevent the formation surrounding the wellbore from cracking, and to prevent drilling fluid from entering the surrounding formation. Thus, downhole pressures are typically maintained within a given range.
The downhole drilling operation, known pressure conditions and the equipment itself may be manipulated to facilitate downhole measurements. It is desirable that techniques be provided to take advantage of the drilling environment to facilitate downhole measurements of parameters such as annular pressure and/or pore pressure. It is further desirable that such techniques be capable of providing one or more of the following, among others, adaptability to various wellbore and/or equipment conditions, measurements close to the drill bit, improved accuracy, simplified equipment, detection of sticking risks, real time data, and/or measurements during the drilling process. Added benefit would be achieved where analysis of wellbore operations could be conducted even in cases where accuracy of measurements and/or readings are poor.